Acetonitrile electrochemical potential shift

The results obtained in both acetonitrile and dichloromethane solutions are similar. Normally, the voltammograms included two reversible or quasi-reversible waves with a Tomes criterion AE = E3/4 - Ei/4, characterizing the reversibility of electrochemical processes [335], of ca 60 mV and a distance between them of 855 20 mV. The first and more intensive wave was attributed to the oxidation of two independent ferrocenylboron caps. A substantial (100 200 mV) cathodic shift of this wave versus the Fc /Fc potential is caused by the donor effect of the macrobicyclic substituent in cyclopentadienyl rings. 

Another class of mixed-metal anion receptors has been investigated which possess redox reporter groups based on two different metal complexes. This enables the quahtative comparison of their comparative anion-sensing abih-ties. Macrocycles 35 and 36 combine the Ru (bpy)3 moiety with a bridging ferrocene or cobaltocenium imit [29]. Electrochemical experiments in acetonitrile solution revealed that the Ru VRu redox potential was insensitive to anion binding, whereas the ferrocene/ferrocenium (in 35) and cobal-tocene/cobaltocenium (in 36) redox couples were shifted cathodically (by 60 mV and 110 mV respectively with chloride). However, the first reduction of Ru°(bpy)3, a Hgand-centred process based on the amide substituted bipyridyl, was also found to imdergo an anion induced cathodic shift (40 mV and 90 mV with chloride for 35 and 36, respectively). 

The oxidation of OH by [Fe(CN)6] in solution has been examined. Application of an electrical potential drives the reaction electrochemically, rather than merely generating a local concentration of OH at the anode, as has been suggested previously, to produce both O and [Fe(CN)6] in the vicinity of the same electrode. With high [OH ] or [Fe(CN)6] /[Fe(CN)6] ratio, the reaction proceeds spontaneously with a second-order rate constant of 2.2 x 10 M s at 25 °C. Under anaerobic conditions, iron(III) porphyrin complexes in dimethyl sulfoxide solution are reduced to the iron(II) state by addition of hydroxide ion or alkoxide ions. Excess hydroxide ion serves to generate the hydroxoiron(II) complex. The oxidation of hydroxide and phenoxide ions in acetonitrile has been characterized electrochemically " in the presence of transition metal complexes [Mn(II)L] [M = Fe,Mn,Co,Ni L = (OPPh2)4,(bipy)3] and metalloporphyrins, M(por) [M = Mn(III), Fe(III), Co(II) por = 5,10,15,20-tetraphenylpor-phinato(2-), 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphinato(2-)]. Shifts to less positive potentials for OH and PhO are suggested to be due to the stabilization of the oxy radical products (OH and PhO ) via a covalent bond. Oxidation is facilitated by an ECE mechanism when OH is in excess. 

This behavior is typical of simple quinones in aprotic solution. " However, the electrochemical behavior of 2,3,5-TMHQ and a-tocopherylquinone in acetonitrile is altered considerably by the addition of a weak acid such as ethyl malonate (Figure 19). Thus, the peak Ic process remains unchanged but reduction peak lie broadens and shifts towards more positive potentials. In addition peak Ila, corresponding to electrooxidation of the quinone dianion... 

The electrochemical behavior of 8 and 9 was explored by cyclic voltammetry in acetonitrile (Fig. 47.6). In the absence of a base, the one-electron oxidation of 8 occurred at 0.42 V. In agreement with the behavior of the corresponding phenolic analogs, this electrochemical step featured the reversible oxidation of the ferrocene group (Fc) to ferricenium (Fc+). In the presence of an imidazole excess, two oxidation steps were observed. A nearly two-electron irreversible oxidation peak appeared, the potential of which shifted toward less positive values as the imidazole concentration increased. Another peak located at 0.52 V featured a one-electron reversible oxidation process, its potential being independent of base concentration. The cyclic voltammograms obtained with 9 under the same conditions were akin to those for 8 (Fig. 47.6b). Except for the fact that no reduction wave featuring the reduction of a transient a-cation, the behavior was extremely similar to that described above for the mono-phenol. 

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